This invention relates to an automatic gain control circuit adapted to be utilized in an AM receiver, in which a variable capacitance diode (hereinafter referred to as "varicap diode") is employed.
Generally, an AM receiver has a ferrite bar antenna for effectuating the signal feeding operation when radio waves are received without an external antenna. A winding for the external antenna is utilized in the case of weak electric fields. The bar antenna has a primary winding for the external antenna which is low in impedance, a secondary tuning winding and a tertiary winding for injection into an IC or an amplifier. The number of turns of the primary and the tertiary windings are approximately equal. The number of turns of the secondary tuning winding is approximately ten times as many as that of the primary or the tertiary winding. Accordingly, the radio wave fed from the external antenna is induced in the secondary tuning winding as a high frequency signal stepped up about ten times.
In the case where a varicap diode is employed in the tuning circuit of the AM receiver as a capacitance element and the receiver circuit is tuned by the varicap, the secondary tuning winding, etc., a problem is encountered. Since the input signal fed from the external antenna is stepped up about ten times and then applied to the tuning circuit, the reversely biased varicap diode acting as a capacitance element is biased in the forward direction, thereby losing its ability to function as a capacitance element when a large input signal is fed from the external antenna. As a result, phenomena such as detuning and oscillation occur so that tuning performance significantly deteriorates.
Conventionally, in order to prevent this problem, a change-over switch switchable between near distance or far distance is provided in the external antenna circuit. Alternatively, an RF amplifier, including an FET, is connected in the external antenna circuit, with the FET gate connected to the tertiary winding to automatically control the gain of the external antenna. It is difficult, however, to automate the switching operation of the change-over switch, and it is necessary to perform the switching operation every time a strong electric field or a weak electric field is received. Furthermore, in an electronic type attenuator employing an FET amplifier, a large amount of attenuation cannot be obtained and it is difficult to prevent the characteristic deterioration caused by a large input.